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5.8  Why is adsorption always exothermic ?

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Adsorption is always exothermic because-

  1. During adsorption, the residual forces of the surface always decrease. As a result, there is a decrease in surface energy, which emits as a heat.
  2. \Delta H Of adsorption is always negative. The freedom of movement of adsorbate (gas) molecule becomes restricted when the adsorbent adsorbs the adsorbate(gas). As a result, the entropy decreases (\Delta S is negative). For the process to be spontaneous \Delta G should be negative.\Delta G=\Delta H-T\Delta S .Since \Delta S is negative, \Delta H has to be negative, so that \Delta G is negative. Hence adsorption is always exothermic.

 

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